Turning to FIG. 1, an example of a conventional class D PA 100 that is capable of amplitude modulation can be seen. This PA 100 generally comprises drivers 102-1 to 102-N (which are typically inverters), capacitors CI1 to CIN, and a matching network 104. In operation, a predetermined number of capacitors (i.e., capacitor CI1 to CIn) are switched, while the remainder (i.e., capacitors CI(n+1) to CIN) are left “off.” This generates an effective capacitance CIEFF that is the sum of the switched capacitor (i.e., CIEFF=CI1+ . . . +CIn) and an effective supply voltage VEFF (i.e., VEFF=VDD*(n/N)). A functionally equivalent circuit having an inverter 106 that represents the drivers 102-1 to 102-N and that includes the effective capacitance CIEFF and effective supply voltage VEFF can be seen in FIG. 2. This allows an output radio frequency (RF) signal RFOUT to be generated from an input RF signal RFIN (which can be amplitude modulated) so as to be applied to the load RL.
A problem with this configuration, however, is that the switching losses is a function of the effective capacitance CIEFF and frequency. These losses can be so significant so as to be prohibitive for many applications, and the efficiency can be very low. In particular, the efficiency of PA 100 may only reach about 54%. Therefore, there is a need for an improved class D PA.
Some other conventional circuits are: Yoo et al., “A Switched-Capacitor Power Amplifier for EER/Polar Transmitters,” IEEE Intl, Solid-State Circuits Conference Digest of Technical Papers (ISSCC) 2011, Feb. 20-24, 2011, pp. 428-430; U.S. Pat. No. 6,882,829; U.S. Pat. No. 7,509,102; U.S. Pat. No. 7,733,187; and U.S. Pat. No. 7,831,227.